Synthesis and Crystal Structure of a Novel Chiral 3D Metal-organic Framework Based on an N-methyl Substituted Salan Ligand①

ZHANG Fng-Wei ZHOU Yn-Fng DONG Jin-Qio LIU Bi-Zhn ZHENG Si-Jing② CUI Yong, ② (School of Chemistry nd Chemicl Technology nd Stte Key Lortory of Metl Mtrix Composites, Shnghi Jio Tong University, Shnghi 200240, Chin)

b (China Tobacco, Shanghai 200240, China)

1 INTRODUCTION

Metal-organic frameworks (MOFs)[1]bearing regular pores and active sites have drawn much attention for the last decade. A vast variety of such materials have been synthesized with their unique properties such as separation, catalysis and gas storage abilities investigated[2]. Among the most privileged of them are the metallosalen based MOFs which have been synthesized with various applications[3～7]in recent years. Derivatives of salen ligands, such as salan, salalen and N-methyl substituted salan ligands have also been developed[8].The ONNO coordinating pockets of these reduced salen ligands are much more flexible thus affording complexes with various configurations. Chiral N-methyl substituted metallosalan complexes tend to wrap around an octahedral metal center in a fac-fac mode, and often give Δ and Λ geometry diastereomer mixtures in solutions. Such nature of N-methyl metallosalan complexes indicates that the chirality of the ligands fails to dictate to the chirality of the metal atoms, and strongly restricts their asymmetric applications[9～11]. We have recently reported a series of MOFs based on chiral salan ligands with their properties surveyed[12～13]. Herein we present the assembly of a chiral metal-organic framework built from an N-methyl substituted salan ligand, which possesses a three-fold interpenatrated structure with all N-methyl metallosalan units taking Δ geometry.

2 EXPERIMENTAL

2. 1 Materials and measurements

All of the reagents involved are commercially available and used without further purification.Elemental analyses of C, H, S and N were performed with an EA1110 CHNS-0 CE elemental analyzer.The IR (KBr pellet) spectra were recorded(400～4000 cm–1region) on a Nicolet Magna 750 FT-IR spectrometer. Power X-ray diffraction patterns of the samples were recorded on a PANalytical X?Pert Pro diffractometer. Thermogravimetric analysis (TGA) was carried out on a STA449C integration thermal analyzer in N2atmosphere at a heating rate of 20 ℃·min–1. CD spectra were recorded on a J-815 spectropolarimeter(Jasco, Japan).

2. 2 Synthesis of 1

The ligand H2L was synthesized via similar procedure according to previous literatures[11,14]. A mixture of Cd(OAc)2(32 mg, 0.12 mmol), H2L(67.6 mg, 0.1 mmol), DMSO (13.5 mL) and H2O(0.7 mL) was sealed in a 50 mL screw-capped vial and heated at 80 ℃ for two days. The vial was then cooled to room temperature and dark-red hexagonal thin plate crystals were collected. The crystals were washed with ethanol and dried in air. Yield: 45.8 mg,51.8%. Elemental analysis for C42H58CdN8O4S:Calcd. (%): C, 57.10; H, 6.62; N: 12.68, S: 3.61.Found (%): C, 56.21; H, 6.51; N, 12.84, S: 3.77. IR(KBr plate, cm–1) 3436(b), 2949(s), 2863(w),1587(s), 1476(m), 1436(w), 1346(m), 1310(s),1283(m), 1166(s), 1151(m), 1132(m), 1058(w),1009(m), 995(m), 909(w), 899(w), 845(w), 823(m),627(w), 613(w), 552(w), 532(w), 509(m).

2. 3 Crystallographic measurements and structure determination

A suitable crystal with dimensions of 0.16mm ×0.14mm × 0.07mm of 1 was carefully glued at the tip of a fine glass fiber with cyanoacrylate adhesive under a polarizing microscope. The X-ray diffraction intensity data were collected with a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochromatized Cu Kα radiation (λ =1.54178 ?) using an ω-φ scan mode in the range of 4.18≤θ≤68.11o(–13≤h≤14, –13≤k≤9, –56≤l≤59) at 293(2) K. A total of 21307 reflections were observed with 4029 independent ones (Rint= 0.0421),and 3901 were observed with I ＞ 2σ(I). The structure was solved by direct methods with SHELXS-97 program[15]and refined with SHELXL-97[16]program by full-matrix least-squares techniques on F2.All non-hydrogen atoms were refined anisotropically,and all hydrogen atoms were included in the final cycle of refinements on the calculated positions bonding to their carrier atoms except for the guest H2O molecule, around which no suitable peaks on the electron density map were found. The final R =0.0313, wR = 0.0773 (w = 1/[σ2(Fo2) + (0.0294P)2+3.9504P], where P = (Fo2+ 2Fc2)/3), S = 1.041,(Δ/σ)max= 0.001, (Δρ)max= 0.355 and (Δρ)min=–0.390 e/?3. The Flack factor is 0.024(9). The selected bond lengths and bond angles for 1 are given in Table 1. No classical hydrogen bonds were found.

3 RESULTS AND DISCUSSION

3. 1 Synthesis and characterization

The chiral N-methyl substituted salan ligand H2L was prepared by Schiff base condensation of(E)-3-(tert-butyl)-2-hydroxy-5-(pyridin-4-yldiazenyl)benzaldehyde and enantiopure trans-1,2-diaminocyclohexane followed by reduction and reductive amination. Assembly of H2L and Cd(OAc)2in DMSO and water (20:1) at 80 ℃ afforded dark red hexagonal thin plate crystals of [Cd L]·DMSO·H2O(1). 1 is stable in air and insoluble in water and common organic solvents. The formula of 1 was verified by elemental analyses, IR and thermo-gravimetric analysis (TGA). As shown in Fig. 1, the phase purity of bulk sample was established by comparison of its observed and simulated PXRD pattern. TGA revealed that the framework is stable up to about 345 ℃ (Fig. 2). The CD spectra of (R)-1 and (S)-1 prepared from (R, R) and (S, S)-H2L respectively are mirror images of each other,demonstrating their absolute configuration and enantiopurity (Fig. 3).

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Fig. 1. Experimental and simulated powder XRD patterns of 1

Fig. 2. Thermal analysis curve of 1

Fig. 3. CD spectra for (S)-1 and (R)-1

3. 2 Structure description

Single-crystal X-ray diffraction reveals that (R)-1 crystalizes in the chiral hexagonal space group P6522, with half of the ligand, half DMSO and half H2O in the asymmetric unit. The Cd center adopts a distorted octahedral geometry and coordinates to the ONNO pocket of one ligand and two pyridyl terminals from two other ligands in cis fashion with a N–Cd–N angle of 89.36(17)°. The ONNO pocket adopts fac-fac and Δ geometry, as shown in Fig. 4(a).Adjacent metal centers are bridged by half of the ligand and adopt a distorted tetrahedral geometry, as shown in Fig. 4(b). The basic building unit (Cd L)1/2runs head-to-tail along the c axis with a Cd–Cd–Cd angle of 132.9oto form double antiparrel helical chains (dotted in red and green) associated with crystallographic 2 axis. As shown in Fig. 5, the wide pitch between the double helical chains is 34.30 ?,which is 2/3 of the length of c-axis, while the narrow pitch is 17.15 ?, 1/3 of the length of the c-axis. Two of the (Cd L)1/2branches from one Cd center belong to one helical chain, and other two branches stretch in the pitches to connect with the same branches from the other helical chains (dotted in purple and yellow). All of the linking (Cd L)1/2units between the helical chains are also part of the adjacent helical chains, and all of the tert-butyl groups point in the helical axis. Space packing unveils three sets of 3D frameworks, penetrating each other to form the whole 3D structure. The helical chains of one set penetrate through the wide pitches of the other two sets alternatively. As shown in Fig. 6, the simplified network indicates that symmetric operation of crystallographic 65axis can render anyone set of the 3D framework unchanged while exchanges the other two sets. The symmetric operation of crystallographic 2 axes still remains all the three individual sets of 3D frameworks, as depicted in Fig. 5(b).

Fig. 4. Coordination environment for Cd atom (Hydrogen atoms and guest molecules are omitted for clarity).(a) Simplified distorted tetrahedral network constructed by neighboring Cd centers,(b) Arrows point from phenolic groups to the pyridyl groups

Fig. 5. Double helices in one set of the 3D frameworks:space packing view (a) and schematic view (b)

Fig. 6. Three-fold interpenetrated structure of 1

4 CONCLUSION

In summary, we have synthesized a chiral N-methyl substituted salan ligand (H2L) and used it as building block to construct a Cd(II) based threefold interpenetrated 3D framework (1). Its structure has been characterized by elemental analyses, IR,TGA, powder XRD and single-crystal X-ray crystallography, respectively. All of the Cd L units in 1 adopt fac-fac and Δ geometry.

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